bug#50005: 28.0.50; silence a couple byte-compiler warnings in ERC

[Mattias Engdegård]

>  (defcustom erc-lurker-ignore-chars "`_"
>    "Characters at the end of a nick to strip for activity tracking purposes.
> -
> +The usual rules regarding ]^\\ and - in \"character alternatives\ apply.

Oops (two typos), but thanks for illustrating the problems with:

> +      (string-trim-right nick (concat "[" erc-lurker-ignore-chars "]+"))

Attempting to construct character alternatives by hand is usually a mistake; 
it's just too error-prone. Better use rx or regexp-opt instead:

 (rx-to-string `(+ (in ,@(string-to-list erc-lurker-ignore-chars))) t)

or

 (concat (regexp-opt (mapcar #'char-to-string erc-lurker-ignore-chars)) "+")